1. Field of the Invention
The present invention relates to an electronic siren and in particular to an electronic siren circuit having the signal generating and power amplifier contained in a first unit and a control head contained in a second unit located remotely from the first, the two units being interconnected by a multiplexing circuit which enables control of all functions of the siren from the remotely located control head through a communication conductor.
2. Description of the Prior Art
Electronic siren circuits such as that disclosed in U.S. Pat. No. 4,189,718 issued to William H. Carson et al and assigned to the assignee of this invention, are widely used, and typically include a waveform generating circuit which generates a manually selected one of a plurality of signals having predetermined frequency envelopes which are in turn used to control the frequency of a square wave generating circuit. The combination of the waveform generating circuit and a high power amplifier circuit in conjunction with a loud speaker produces the various siren sounds commonly referred to as a wail, yelp, and two-tone. Additionally, such circuits now frequently include circuitry which enables the siren circuit to function as a public address amplifier for a microphone carried in the vehicle to which the siren is attached, circuitry for coupling a vehicle's radio receiver to the siren amplifier so that incoming two way communications can be monitored from outside the vehicle, and circuitry for simulating high volume air horns typically carried in vehicles such as fire trucks.
Such circuits are designed to operate at relatively high power levels. Simultaneously, since such siren circuits are typically used in mobile vehicles, the available power for operating the circuits is limited thereby necessitating high efficiency circuitry. While modern electronic technology has enabled such circuits to be fabricated in relatively compact units, the continuing reduction in size of modern motor vehicles has imposed substantial and continuing reductions in the available space for mounting the siren circuits on or near the dashboard of the vehicle or similar locations convenient for control of the circuit. It has further been impractical to mount such a siren circuit in a position in the vehicle remote from the dashboard inasmuch as the circuit must be manually operated, for example, to select or control the desired siren tone.
To overcome the space problem, the siren has been packaged in two separate units, one relatively small, function selecting unit for mounting on the dash board and the other containing the power portion for mounting remotely, such as in the trunk of the car. Interconnecting conductors serve in communicating function selection between the units. In such a prior art arrangement, an electrical signal is transmitted over the conductors, this signal changing its characteristic for each selected function: this signal utilizes voltage changes between discrete functions. A problem arises by reason of extraneous voltages being induced into the conductor from such sources as a radio transmitter in the same vehicle, which causes unwanted voltage jumps in the signal and consequent accidental shifts in function selection.